20 April 2012

The polar bear is the world's largest land carnivore. It is also the largest bear. An adult male weighs around 350–680 kiloggrams (770–1,500 lb),while an adult female is about half that size.

The polar bear is native to the Arctic Circle.

Compared to its close cousin, the brown bear, the polar bear has many body characteristics adapted for cold temperatures, for moving across snow, ice, and open water, and for hunting the seals which make up most of its diet.

Polar bears are born on land but spend most of their time in the waters hunting seal. Their scientific name (Ursus maritimus) means "maritime bear", and derives from this fact. Polar bears can hunt their preferred food of seals from the edge of sea ice, often living off fat reserves when no sea ice is present.

Polar bears older than previously thought

Polar bears diverged from their closest relatives about 600,000 years ago, according to a new genetic study published in the April 20 issue of the journal Science.

The findings suggest the cold adapted species is about five times older than previously thought, and may have had more time to adapt to arctic conditions than recently assumed. Previous studies of polar bears focused mainly on mitochondrial or mtDNA, which is passed on from mother to offspring and only comprises a very small portion of the entire genome.
Video: Polar Bears

Because each part of the genome can tell its own story, using solely mtDNA to reconstruct a species’ evolutionary history is like reading only a few pages of a book -- scientists are prone to miss large chunks of information.

In this study, Frank Hailer from Uppsala University in Sweden and colleagues set out to test whether the nuclear genome told the same story as mtDNA, which indicated that polar bears are essentially a recently evolved type of northern brown bear.

Their results showed the opposite: data from many independently inherited regions of the nuclear genome revealed that both polar and brown bears are much older, as species, than previously suggested. At the time the bears diverged, the Pleistocene climate record shows that global temperatures reached a long-term low.

This could be purely coincidental, the authors argue, but these results suggest that the drastic climatic cooling events during the Pleistocene period were associated with the evolutionary origin of the polar bear. As human activities continue to speed up the rate of climate change, the arctic could reach higher temperatures more rapidly than in previous warm phases.

This study suggests that past adaptation to a changing climate may have been a slow process. Consequently, polar bears may not have enough time to adjust to these warmer conditions as they have in the past.